1. Field of the Invention
The present invention relates to a musical tone generation apparatus applied to a frequency modulation technology in an electronic musical instrument.
2. Description of the Prior Art
Conventionally, an FM operator as a sound source utilizing a frequency modulation system generates many partial tones or harmonic overtones by arithmetic operations of simple equations, and is widely known as one effective method of synthesizing percussion tones, and wind instruments tones, and the like. In this FM operator, a waveform memory storing waveform data of musical tone signals is accessed by an address signal of a desired repeating frequency to read out waveform data, and a musical tone signal is generated based on the readout waveform data.
As a developed type of this FM operator, an operator which feeds back an amplitude value of readout waveform data to an address input of a waveform memory at a proper feedback rate to modulate a read address, thereby obtaining various waveforms, a sawtooth waveform, for example, different from the waveform data stored in the waveform memory is known (Japanese Patent Publication No. Sho 61-20875).
In addition, a musical tone signal forming apparatus which prepares various frequency modulation algorithms (combinations of modulations and additions among operators) to obtain a variety of musical tones, and time-divisionally shares one fundamental waveform memory to be able to perform arithmetic operations for a plurality of operators is also disclosed (Japanese Patent Publication No. Sho 64-4199).
However, in the above-mentioned prior arts, when processing for, e.g., removing some harmonic overtones from a musical tone generated by a given FM term (a combination of a carrier and a modulator) is to be executed, a filter must be separately used. When musical tones are synthesized by selecting a proper algorithm, depending on the frequency relationship of operators, a musical tone having a rectangular waveform can be obtained, but it is difficult to change the rate of the rectangular width in one cycle of the waveform. Moreover, it is difficult to modulate the rectangular width. Generally, such modulation is called PWM (Pulse Width Modulation).